PMN-MDSCs promoted CIA severity through modulating B cells
To verify that PMN-MDSCs played a proinflammatory or immunosuppressive
role in CIA, we depleted PMN-MDSCs in CIA mice from day 32 (disease
progression stage) using anti-Ly6G antibodies (1A8), and the control
group used the isotype IgG Ab, The Ly6G Ab was effective in reducing the
PMN-MDSC ratio and counts in the joints and spleens (Fig. S2A, S2B).
PMN-MDSC depletion decreased the severity of
arthritis,
maintaining an average arthritis score of < 6 (Fig. 2A). Ly6G
Ab alleviated joint swelling and decreased inflammatory cell
infiltration and synovial hyperplasia compared to control isotype IgG
(Fig. 2B). PMN-MDSCs depletion also decreased IgG and
TNF-𝛂
in the plasma (Fig. 2C). And surprisingly, we found that the frequencies
and absolute number of
TNF-𝛂+B
cells and Ki67+B cells performed by flow cytometry
also decreased significantly in the joints and spleens after PMN-MDSCs
were removed (Fig. 2D-2H). And the
TNF-𝛂
transcriptional level in the joints and spleens detected by RT-qPCR also
declined in Ly6G Ab group (Fig. S2C, S2D).
However, the anti-Ly6G antibody could deplete all cells with the surface
marker Ly6G, and its potential neutralizing effect on neutrophils could
not be ruled out. To rule out this possibility, PMN-MDSCs isolated from
the spleens of CIA mice (clinical score >6) on day 35 were
adoptively transferred to recipient CIA mice on days 21 and 28. The
transferred PMN-MDSCs aggravated arthritis severity (Fig. 3A). The
swelling of the joints was amplified and the pathology showed increased
immune cell infiltration, inflammatory exudation, cartilage damage, and
bone erosion (Fig. 3B). The concentration of IgG increased significantly
in the plasma after PMN-MDSCs transfer, the TNF-𝛂 also increased
slightly although had no significant difference (Fig. 3C). In addition,
PMN-MDSCs transfer enhanced the frequencies and absolute number of
TNF-𝛂+B cells and Ki67+B cells in
the joints and spleens in vivo (Fig 3D-3H). Therefore, the
proinflammatory role of PMN-MDSC in the pathogenesis of CIA were
validated through selective depletion and adoptive transfer experiments,
and B cell responses were modulated by PMN-MDSC.